Apparatuses for determining a property of a heart are used, for example, in the field of interventional treatment for atrial fibrillation. During the interventional procedure cardiac tissue is denaturized by applying energy. For example, in radio frequency (RF) catheter ablation radio frequency energy is applied to the cardiac tissue by an ablation catheter, wherein due to resistive losses in the tissue the myocardium is heated up and the heated muscle cells in the cardiac tissue die off and lose their biological function. Since the optimal parameters for ablation, such as power and duration, vary largely due to sizeable intra- and inter-patient differences in thickness of the local heart wall, perfusion, blood pressure, etc., it is important to be able to monitor the progress of the lesion development in the tissue and, preferentially, the depth of the lesion with respect to the thickness of the tissue at the treatment site, to prevent damages to the patient caused by underheating or overheating of the ablation site. Ablation catheters that are used in practice therefore provide feedback about properties of the heart, including, for example, local conduction (electrograms), tissue impedance and/or temperature.
US 2004/0181139 A1 discloses an apparatus for determining a property of a heart. The apparatus comprises a catheter for electrical ablation of a vessel/heart and for monitoring the quality of the lesion by determining the tissue impedance or the electrical signal strength at the ablation site. These local properties of the heart, however, only provide limited information about the progress of the lesion and, in particular, about the depth of the lesion with respect to the thickness of the tissue at the treatment site.